Known fuel cell stacks include a solid oxide fuel cell (hereinafter, may be referred to as “SOFC” or merely as “fuel cell”) stack which uses a solid oxide as an electrolyte. The SOFC stack is, for example, a stack of a number of fuel cells, each having an anode and a cathode provided on respective opposite sides of a plate-like solid electrolyte layer. Fuel gas (e.g., hydrogen) and oxidizer gas (e.g., oxygen in air) are supplied to the anode and the cathode, respectively, for their chemical reaction through the solid electrolyte layer, thereby generating electricity.
Generally, the fuel cell is connected, for use, to a separator which separates a fuel gas section and an oxidizer gas section. This connection is usually established through a joint formed of a brazing filler metal such as Ag brazing filler metal, whereby fuel gas and oxidizer gas are separated from each other.
The following technique is disclosed for joining the fuel cell and the separator. The disclosed technique uses a glass sealing material for providing a seal between the separator and the fuel cell (refer to Patent Document 1).
According to another disclosed technique, a refractory metal and ceramic are brazed together in the atmosphere by use of Ag brazing filler metal to which a nonreducing oxide such as Al2O3 is added (refer to Patent Document 2). A further technique discloses a glass material which contains Al2O3, for use with the SOFC (refer to Patent Document 3).
Meanwhile, in some cases, sufficient reliability is not necessarily secured in joining the fuel cell and the separator by use of glass or a certain brazing filler metal. For example, in joining by use of Ag brazing filler metal only, for a structural reason, an Ag brazing filler metal joint is disposed at the boundary between oxidizer gas and fuel gas. Thus, the following possibility exists: in use over a long period of time, component atoms (oxygen atoms) of oxidizer gas and component atoms (hydrogen atoms) of fuel gas enter the Ag brazing filler metal joint from the oxidizer gas section and the fuel gas section, respectively, and diffuse and react in the Ag brazing filler metal joint, whereby voids (pores) are generated in the Ag brazing filler metal joint, resulting in leakage of the gases.
Techniques for preventing the generation of voids are disclosed (refer to Patent Documents 4 and 5). Through use of various Ag alloys having low gas diffusion speed, the life of a brazing filler metal can be prolonged.
However, although the techniques disclosed in Patent Documents 4 and 5 can prolong the life of the joint (fuel cell), it is not easy for them to ensure a practically sufficient life of tens of thousands of hours.